Aluminum-silicon alloys containing less than about 11.6% by weight of silicon are referred to as hypoeutectic alloys and have seen extensive use in the past. The unmodified alloys have a microstructure consisting of primary aluminum dendrites, with a eutectic composed of acicular silicon in an aluminum matrix. However, the hypoeutectic aluminum-silicon alloys lack wear resistance.
On the other hand, hypereutectic aluminum-silicon alloys, those containing more than about 11.6% silicon, contain primary silicon crystals which are precipitated as the alloy is cooled between the liquidus temperature and the eutectic temperature. Due to the high hardness of the precipitated primary silicon crystals, these alloys have good wear resistant properties, and while alloys of this type have good fluidity, they have a relatively large or wide solidification range. The solidification range, which is a temperature range over which the alloy will solidify, is the range between the liquidus temperature and the invariant eutectic temperature. The wider the solidification range, the longer it will take for an alloy to solidify at a given rate of cooling. Thus, for casting purposes, a narrow solidification range is desired.
As a general rule, hypereutectic aluminum-silicon alloys are more difficult to case than hypoeutectic aluminum-silicon alloys because hypereutectic aluminum-silicon alloys are difficult to "feed" and this casting characteristic worsens as the silicon content increases. Thus, hypereutectic aluminum-silicon alloys containing 16% to 20% silicon have greater opportunity of finding commercial use than hypereutectic aluminum-silicon alloys containing more than 20% silicon (such as the class containing 21% to 25% silicon) because they have a narrower solidification range which produce less shrinkage microporosity and inherently a smaller primary silicon particle size. The smaller primary silicon size results in better machinability and is advantageous in wear applications. The absence of shrinkage microporosity in the cylinder bores of 4-stroke internal combustion engines is essential for low oil consumption.
Typical wear resistant aluminum-silicon alloys are described in U.S. Pat. No. 4,603,665. U.S. Pat. No. 4,603,665 describes a hypereutectic aluminum- silicon casting alloy having particular use in casting engine blocks for marine engines. The alloy of that patent is composed by weight of 16% to 19% silicon, 0.4% to 0.7% magnesium, less than 0.37% copper, and the balance aluminum. The alloy has a narrow solidification range providing the alloy with excellent castability, and as the copper content is maintained at a minimum, the alloy has improved resistance to salt water corrosion.
It has also been recognized in the metallurgical field that the magnesium content of the aluminum silicon alloy should be maintained below its solubility limit, because there is no heat treatment response benefit in going to a higher magnesium content. Moreover, a higher magnesium content also has an adverse effect on melt handling, as well as producing a decrease in fluidity, which can contribute to making castings with defects.
The solubility limit of magnesium in an aluminum-silicon alloy varies with the chemistry. For example, the solubility limit of magnesium in hypoeutectic aluminum-silicon alloys that do not have other alloying elements, other than magnesium, is 0.80%. However, the addition of other alloying elements, such as copper. manganese and iron, to the alloy can reduce the solubility limit of magnesium to a value of about 0.70%.
If the magnesium content is above the solubility limit, the compound Mg.sub.2 Si is produced, which results in increased brittleness in the alloy, and it is well recognized that the insoluble Mg.sub.2 Si phase should be avoided. For example, the 9th Edition of The Metals Handbook, Vol. 15, September 1989, p.746, states "The hardening phase Mg.sub.2 Si displays a useful solubility limit corresponding to approximately 0.70% magnesium, beyond which no further strengthening occurs or matrix softening takes place".
Evaporable foam casting is a known technique, in which a pattern is formed of an evaporable polymeric material, such as polystyrene, having a configuration substantially identical to the part to be cast. The pattern is normally coated with a ceramic wash coat, which prevents metal sand reaction and facilitates cleaning of the cast metal part. The pattern containing the wash coat is supported in the mold and surrounded by an unbonded particulate medium, such as sand. When the molten metal contacts the pattern, the foam material in various fractions, melts, vaporizes, and decomposes with the liquid and vapor products of degradation passing into the interstices of the sand, while the molten metal replaces the void created by vaporization of the foam material to thereby form a cast article identical in shape to the pattern. In the evaporable foam casting art, it has generally not been recognized that the molten metal is in direct contact with the liquid foam decomposition products for a significant portion of the process, and therefore, if the molten metal is reactive enough due to its alloy content of reactive elements, the molten metal can react with the liquid foam products and alter the resulting volume fraction of reaction products.
In an evaporable foam casting process, it is desirable to slow the mold filling process by the permeability of the coating on the foam to provide ample time for the elimination of vapors generated by the decomposition of the pattern from the molten alloy. It has been found that when casting large articles, such as engine blocks, a defect commonly referred to as a "liquid styrene defect" can occur with hypereutectic aluminum-silicon alloys, that are not necessarily found with hypoeutectic aluminum-silicon alloys. The defect appears as elongated rifts and may extend through the thickness of the casting. It is believed the liquid styrene defect results because the liquid styrene that accumulates on the advancing metal front stays liquid longer than the metal, particularly when two molten metal streams meet in the far reaches of a complex casting and have lost a significant portion of their initial super heat. Even after solidification, the solidified metal continues to transfer heat to the liquid styrene, eventually causing its evaporation and creating a void in the space previously occupied by the liquid styrene. As an engine block is subjected in use to internal pressures, leakage can occur through the defect.
Attempts have been made in the past to eliminate this liquid styrene defect by using coatings of different permeabilities and by increasing the temperature of the molten alloy, but these attempts have not eliminated the defect. It has also been suggested to remove the copper from aluminum alloy 390 (16%-18% silicon, 4.5% copper), to eliminate the bottom half of the solidification range of that alloy and narrow the solidification range, but again, a change in the solidification range has not had an influence in controlling the liquid styrene defect. The use of reactive ingredients in the wash coating has been suggested, but the attempts have not been successful. The paradigm that exists with the evaporable foam casting process is that there is believed to be a vapor gap between the molten metal and the liquid products of foam decomposition, and therefore no chemical reaction is expected between the molten metal, even if it is reactive, and the liquid foam products of decomposition. In essence, it is generally not accepted that the liquid metal and liquid styrene are in direct contact as described above.